Disk-type data storage systems, and particularly optically readable disk systems, require proper placement of a focused beam of light, such as a laser beam, with respect to the disk. Placement in the axial direction, i.e. towards or away from the disk surface, involving focusing of the light beam and placement in the radial direction, referred to as tracking, are needed to assure that a light beam of the required size and intensity impinges on the disk at the desired location.
Methods for monitoring and controlling tracking and focusing of the beam typically involve detection of characteristics of at least a portion of the beam which is reflected from the disk surface. One such system is described in U.S. Pat. No. 4,446,546 issued May 1, 1984 to Miller. This system uses a quadrature detector which produces an "S" curve for use in focus control.
A persistent problem with such systems is that commonly-used detecting devices are unable to distinguish between characteristics which result from defocusing or track crossing and characteristics which result from other changes in the optical system such as movement or misalignment of the optical axis of the reflected beam or misalignment of optical components of the system. Because of this inability to distinguish, a misalignment of some components of the optical system can result in the detector system producing a focus control signal or a tracking control signal which is erroneous, i.e. which causes the focused point of the light beam to be positioned away from the desired position.
A number of methods have been devised to overcome or compensate for the lack of signal discrimination. If the optical detectors are relatively small compared to the size of the beam being detected, movement of the beam axis with respect to the detectors may leave the detectors substantially within the beam and thus substantially unaffected by such movement. However, for many focus correction systems, the beam being detected must be focused on the detector. Producing a focused spot which is sufficiently large to avoid the effects of axis movement during the anticipated life of the device places constraints on other components, such as lens apertures and bit density, which make this solution impractical and expensive to implement.
The tracking system can be separated from the focus system by, for example, a beam splitting arrangement intended to provide a relatively error-free tracking system. However, by splitting the optical path into two optical paths which pass through different optical elements, events such as optical axis movement in one path may not occur in the other path and thus signals related to one optical path may not be useful in correcting errors in the other optical path. Furthermore, this arrangement is beneficial only to the tracking system and does not solve problems associated with the focus system.
Conversely, systems such as that disclosed in U.S. Pat. No. 4,123,652, issued Oct. 31, 1978 to Bouwhuis, include a focusing system which is intended to overcome certain optical faults. However, Bouwhuis does not disclose overcoming optical faults in the tracking system. Furthermore, Bouwhuis requires splitting of the optical path thus creating the possibility for independent optical faults in the two optical paths.
Accordingly, there is a need for a system for tracking and focusing an optical beam which is insensitive to optical faults such as beam axis movement or component misalignment and which does not involve subjecting split beams to independent optical faults.